1. Field of Invention
This invention relates to an adiabatic mold made from a composite material. It is particularly directed to such molds used for the curing of chemically curable compositions.
2. Description of the Related Art
Chemically curable compositions are generally formed into shapes and allowed to cure, or harden, via chemically-induced polymerization and/or crosslinking reactions. The shaping process can be accomplished by molding, such as injection molding, compression molding, cell casting and the like. Sheet materials can be obtained by cell casting (a batch mode) or by continuous casting or molding onto a conveyor belt or belt press.
During the curing process, loss of heat to the surrounding materials and to the environment retards reaction kinetics. This can result in longer curing times and can decrease resulting physical properties. The materials that are used for molds, conveyor belts and other containing structures are typically metals, frequently aluminum or steel, or thick reinforced resin-impregnated fabrics. These absorb heat generated by the chemical curing reaction and generally must be heated or preheated in order to achieve satisfactory curing. Further, the resins are often degraded by heat, and the structures can be used only a limited number of times.
The primary function of a mold is to hold the desired shape while the composition cures. In order to maintain the shape and tolerance, the mold must have sufficient structural rigidity. Typically, support for conventional molds is made up of wood or metal bracing which adds mass, potentially increasing the volumetric heat capacity of the mold. In addition, such typical support increases the complexity of the mold""s construction. The structural complexity is especially visible in molds for making shapes. The complex bracing required often require multiple pieces of, for example, ply wood, constructed to surround the shaped mold to retain the structural integrity of the mold.
This invention is directed to an essentially adiabatic mold made from a composite structure having low thermal conductivity and an exposed shell component having a low heat capacity per unit area, that can be used in the curing of chemically curable compositions.
In one embodiment, the mold is a structure for cell casting sheet or shaped articles from chemically curable compositions. In another embodiment, the mold is a belt for the continuous casting of sheet material from chemically curable compositions.
The mold of the invention are made from a composite structure which comprises an exposed shell component, a barrier shell component, and a honeycomb core interposed therebetween. At least the upper shell of the mold is exposed to the chemically curable composition. The composite structure has a thermal resistance (R value) of at least about 1.0 ft2-xc2x0 F.-hr/Btu (about 0.19 m2-xc2x0 K/W) and preferably about 3.85 ft2-xc2x0 F.-hr/Btu (about 0.677 m2-xc2x0 K/W). In order to retard heat absorption, the exposed shell component should preferably have a thickness that results in a low heat capacity per unit area value for the exposed shell component. Where the curable composition is a methyl(meth)acrylate composition, the heat capacity per unit area is should be less than about 14,500 Joules/xc2x0 K-m2(xe2x80x9cJ/xc2x0 K-m2) preferably less than about 8000 Joules/xc2x0 K-m2[2], more preferably less than about 6000 Joules/xc2x0 K-m2, even more preferably less than about 1400 Joules/xc2x0 K-m2. Low energy absorption of the of the exposed shell component is complementary to thermal resistance of the composite structure making up the mold. In a preferred embodiment, the exposed shell component further comprises a release surface.
The invention is further directed to a method for making a cured article, comprising the steps of (a) placing a chemically curable composition on a mold as described above; (b) allowing the composition to cure, wherein no heat is added to the composition or to the mold; and (c) removing the cured article from the mold.
The invention itself, together with further objects and attendant advantages, will best be understood by reference to the following detailed description, taken in conjunction with the accompanying drawings.